The present invention generally relates to a method and system for cardioverting an atrial tachyarrhythmia. The present invention is more particularly directed to such a system and method which is capable of safely cardioverting an atrial tachyarrhythmia in the presence of atrial pacing.
Atrial fibrillation is probably the most common cardiac arrhythmia. Although it is not usually a life-threatening tachyarrhythmia, it is associated with strokes thought to be caused by blood clots forming in areas of stagnant blood flow as a result of prolonged atrial fibrillation. In addition, patients afflicted with atrial fibrillation generally experience rapid and irregular beating of the heart and may even experience dizziness as a result of reduced cardiac output.
Atrial fibrillation occurs suddenly, and many times can only be corrected by discharging electrical energy into the atria of the heart of the patient. This treatment is preferably synchronized to a detected R wave of the heart in order to avoid shocking the atria during the T wave or vulnerable period of the heart. The amount of energy which may be required to successfully cardiovert the atria can be as low as one joule and as high as six joules. In most cases, energy of about two to four joules is required to cardiovert atrial fibrillation back to normal sinus rhythm (NSR).
Implantable atrial defibrillators are known which detect the presence of atrial fibrillation and provide a single cardioverting pulse of electrical energy to the atria when atrial fibrillation is detected. One such defibrillator disclosed in U.S. Pat. No. 5,207,219 applies the therapy in synchrony with a detected R wave and after a minimum cardiac cycle interval to avoid therapy application during the ventricular vulnerable period of the heart thereby preventing the induction of a lethal ventricular arrhythmia. This therapy has been found to be very effective and safe. It, however, assumes that no other therapy is being applied to the heart at the time that the defibrillator is detecting for a suitable R wave for synchronized therapy delivery.
Some patients, and particularly those with sick sinus syndrome, require continuous atrial pacing because of a dysfunctional Sinus node precluding the production of intrinsic atrial activations or P waves. For those patients, the right atrium is continuously paced at a minimum or rate responsive rate. The pacing is performed in an inhibit mode to inhibit an atrial pacing pulse should a P wave be spontaneously produced by the heart.
To accommodate or treat a patient with sick sinus syndrome who also has episodes of an atrial tachyarrhythmia, such as atrial fibrillation, an atrial cardioverter must be able to both continuously pace the atria and cardiovert the atria in the presence of such pacing. Unfortunately, during atrial fibrillation, the amplitude of atrial activity becomes drastically diminished and intrinsic atrial activity may be undersensed. This essentially reduces the atrial pacing to fixed rate, non-inhibited pacing, commonly referred to as AOO pacing.
As can be appreciated from the above, under these conditions, chaotic heart activity is evident. Not only are the atria being paced at a fixed rate, but R waves, at a rapid and irregular rate are also being produced by the heart. In order to find a safe and effective time to apply tachyarrhythmia therapy, measures not heretofore known must be used. The present invention provides such measures.